The present invention relates to a switching power converter for use in various electronic apparatuses, and more particularly to a synchronous rectifying circuit in the switching power converter.
Technologies regarding synchronous rectifying circuits in conventional switching power converters for use in various electronic apparatuses have been disclosed, for example, in the Japanese Laid-open Patent Application, Publication No. Hei 5-137326. FIGS. 8 and 9 are circuit diagrams of switching power converters disclosed in the Japanese Laid-open Patent Application, Publication No. Hei 5-137326.
A conventional switching power converter shown in FIG. 8 is configured so that an AC input Vin is rectified by a rectifying diode bridge 601 and a DC high voltage (100 V for example) is generated at a smoothing capacitor 602. Energy is stored in and released from the excited inductance of a transformer 604 by a power MOSFET 603 on/off-controlled by a control circuit (not shown). A rectifying diode 620 is connected to the secondary winding of the transformer 604, and the current in the secondary winding of the transformer 604 flows through the diode 620 to charge a smoothing capacitor 606. In addition, the current is smoothed by a smoothing reactor 608 and a smoothing capacitor 607, and a DC voltage is output as a DC output Vout. Since the diode 620 is used in the conventional switching power converter configured as described above, the proportion of a loss owing to the diode 620 in the total loss of the apparatus becomes large when obtaining a DC output of a low voltage (3 V for example), thereby raising a problem.
A conventional switching power converter shown in FIG. 9 has been proposed to solve the problem encountered in the switching power supply circuit shown in FIG. 8. In the switching power converter shown in FIG. 9, instead of the diode 620 shown in FIG. 8, a power MOSFET 705 is connected. In the switching power converter shown in FIG. 9, an N-channel MOSFET is used as the power MOSFET 705. The on/off control of the power MOSFET 705 is carried out by using a voltage generating in the auxiliary secondary winding 704C of a transformer 704. The conventional switching power converter shown in FIG. 9 has a lower conduction loss in comparison with the apparatus comprising the rectifying circuit including the diode shown in FIG. 8, and therefore has high power efficiency in the whole apparatus.
FIG. 10 is a waveform diagram of the synchronous rectifying circuit of the switching power converter shown in FIG. 9. A part (a) of FIG. 10 shows the waveform of a primary current flowing through the power transistor 603 serving as a main switch, a part (b) of FIG. 10 shows the waveform of a voltage generating at the auxiliary secondary winding 704C of the transformer 704, and a part (c) of FIG. 10 shows the waveform of a secondary current flowing through the power MOSFET 705. Synchronous rectifying means that a switching device, such as the power MOSFET 705, is used as a rectifying switch as described above.
A problem arising in the above-mentioned switching power converter is to control the timing of the on/off control of the synchronous rectifying switch highly accurately. For example, if the turn-on timing of the synchronous rectifying switch in the switching power converter shown in FIG. 9 is too early, a large turn-on loss occurs because the voltage of the synchronous rectifying switch is not lowered sufficiently. Conversely, if the turn-on timing of the synchronous rectifying switch is too late, a conduction loss at a body diode inside the synchronous rectifying switch increases. On the other hand, if the turn-off timing of the synchronous rectifying switch is too early, the conduction loss at the above-mentioned body diode increases. Conversely, if the turn-off timing of the synchronous rectifying switch is too late, a period occurs during which the synchronous rectifying switch and the main switch turn on simultaneously. As a result, a large loss owing to a short-circuit current occurs.
In the conventional switching power converter shown in FIG. 9, the turn-off of the synchronous rectifying switch 705 is carried out by the voltage reversion of the auxiliary secondary winding 704C. This voltage reversion takes place when the main switch 603 turns on. Hence, a period occurs during which the main switch 603 and the synchronous rectifying switch 705 turn on simultaneously, although the period is instantaneous. As a result, a large loss owing to a short-circuit current occurs in the conventional switching power converter.
In addition, as a conventional switching power converter of a transformer-insulation type, wherein an AC voltage generated at a secondary winding is synchronously rectified and a power is supplied to a load, apparatuses disclosed in U.S. Pat. No. 5,383,106 and U.S. Pat. No. 5,430,633 are available. Both the switching power converters are flyback converters wherein a series circuit comprising a capacitor and a switch is connected to the primary winding of a transformer. In these apparatuses, when magnetic energy stored in the transformer is released from the secondary winding, an inductance and a capacitor connected equivalently in series with the winding of the transformer cause resonance, and the current flowing through the secondary winding has a resonance waveform. U.S. Pat. No. 5,430,633 discloses a circuit wherein rectifying means connected to the secondary winding is a synchronous rectifier. FIG. 11 is a circuit diagram of a switching power converter with a synchronous rectifier disclosed in U.S. Pat. No. 5,430,633. FIG. 11 simply shows only the main configuration portion of the switching power converter with the synchronous rectifier disclosed in U.S. Pat. No. 5,430,633.
As shown in FIG. 11, in the switching power converter of U.S. Pat. No. 5,430,633, a capacitor 125 and two switches 110 and 120 are connected to the primary winding 132 of a transformer 130. The two switches 110 and 120 comprise transistors 111 and 121 and the body diodes 112 and 122 thereof, respectively. A coil 142 and a capacitor 144 are connected in series with the secondary winding 134 of the transformer 130 so as to produce resonance. Furthermore, a synchronous rectifier 440 having a synchronous rectifying transistor 441 and a body diode 442 is connected to the secondary winding 134 of the transformer 130. This synchronous rectifying transistor 441 is configured so as to be controlled depending on the change of the voltage of the tertiary winding 136 of the transformer 130. Still further, the output Vout of this switching power converter is fed back to the switches 110 and 120 via control means 160.
A problem arising in the switching power converter configured as described above is to control the timing of the on/off control of the synchronous rectifying transistor 441 highly accurately. In the conventional switching power converter shown in FIG. 11, the on/off control of the synchronous rectifying transistor 441 is based on the change of the voltage of the tertiary winding 136. After the first switch 110 (hereafter referred to as the first switch) on the primary side of the transformer 130 turns off, when the voltage of the tertiary winding 136 of the transformer 130 becomes higher than the threshold value of the gate voltage of the synchronous rectifying transistor 441, the synchronous rectifying transistor 441 turns on. Therefore, the turn-on of the synchronous rectifying transistor 441 may become earlier than the turn-on (the start of conduction of the body diode 442) of the synchronous rectifier 440 or the turn-on (the start of conduction of the body diode 122) of the second switch 120 (hereafter referred to as the second switch) on the primary side of the transformer 130. In this case, a turn-on loss occurs because the voltage of the synchronous rectifying transistor 441 is not lowered sufficiently. On the other hand, after the switching transistor 121 of the second switch 120 turns off, when the voltage of the tertiary winding 136 becomes lower than the threshold value of the gate voltage of the synchronous rectifying transistor 441, the synchronous rectifying transistor 441 turns off. For this reason, a period occurs during which a reverse current after a half period of resonance flows through the synchronous rectifying transistor 441, thereby raising a problem of returning a power to the primary of the transformer 130. Furthermore, a period occurs during which the synchronous rectifying transistor 441 and the first switch 110 turn on simultaneously, although the period is instantaneous, thereby raising a problem of a power loss.
The present inversion is intended to solve the above-mentioned problems and to provide a switching power converter capable of optimizing the timing of control for turning on/off a synchronous rectifying transistor.
In order to attain the above-mentioned object, a switching power converter in accordance with the present invention comprises:
a DC input power supply;
a transformer having at least a primary winding and a secondary winding;
a first switch connected in series with the primary winding to form a series circuit, the series circuit being connected in parallel with the DC input power supply;
a second switch connected equivalently across the both ends of the primary winding via a capacitor;
a third switch connected in series with the secondary winding;
a first control drive circuit for alternately turning on/off the first switch and the second switch and having predetermined on-periods, off-periods and rest periods by detecting the output of the series circuit of the third switch and the secondary winding; and
a second control drive circuit for turning on the third switch after a predetermined period from the turn-on of the second switch and for turning off the third switch before a predetermined period from the turn-off of the second switch.
In the switching power converter configured as described above, since the current flowing on the secondary side of the transformer has a resonance waveform, a conduction loss owing to the drive timing of the switch for synchronous rectification occurs less, whereby the current capacity of the diode connected in parallel with the switch can be reduced.
A switching power converter in accordance with another aspect of the present inversion comprises:
a DC input power supply;
a series circuit of a first switch and a second switch connected in parallel with the DC input power supply;
a transformer having at least a primary winding and a secondary winding;
a capacitor connected across both ends of any one of the first switch and the second switch via the primary winding;
a third switch connected in series with the secondary winding;
a first control drive circuit for alternately turning on/off the first switch and the second switch and having predetermined on-periods, off-periods and rest periods by detecting the output of the series circuit of the third switch and the secondary winding; and
a second control drive circuit for turning on the third switch after a predetermined period from the turn-on of the second switch and for turning off the third switch before a predetermined period from the turn-off of the second switch.
In the switching power converter configured as described above, since the current flowing on the secondary side of the transformer has a resonance waveform, a conduction loss owing to the drive timing of the switch for synchronous rectification occurs less, whereby the current capacity of the diode connected in parallel with the switch can be reduced.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.